1. Field of the Invention
This invention is in the field of unsaturated polyesters curable with an unsaturated monomer, and more particularly, is in the field of low smoke density polyesters.
2. Brief Description of the Prior Art
Traditionally, fire-retardant polyesters have been made with chlorinated or brominated materials, or, in some cases, phosphorus-containing materials. These materials may be merely additives to the polyester or they may be actual reactants. Although polyesters prepared with such materials are fire-retardant in the sense that they have low flame spreads and are self-extinguishing, they unfortunately emit a thick, dense smoke when exposed to an open flame. The high smoke levels are, of course, undesirable and often are more serious than the fire itself.
We have found that high smoke levels are due not only to the presence of halogen-containing or phosphorus-containing materials in the polyester, but are also due to aromatic components in the cured polyester. Aromaticity may be introduced into the cured polyester from the crosslinking monomer or from the ingredients used to prepare the polyester itself. An example of an aromatic crosslinking monomer is styrene which is by far the most widely used crosslinking monomer. Examples of commonly used aromatic ingredients used to prepare the polyester are the various isomers of phthalic acid, and Bisphenol A.
We have found that completely aliphatic polyester resins made from unsaturated aliphatic polyesters cured with aliphatic vinyl crosslinking monomers such as methyl methacrylate produce very little smoke when cured thermosets are exposed to an open flame. Unfortunately, many of these all-aliphatic polyester thermosets are weak, having low flexural strength and flexural modulus.
The weakness of polyesters cured with aliphatic monomers such as methyl methacrylate is well known in the art. It is believed that the unsaturated polyesters and methyl methacrylate do not copolymerize readily and, therefore, give a composition in which some of the polyester remains unreacted in a mixture of polymethyl methacrylate and some loosely formed polyester-methyl methacrylate copolymer. See, for example, "Factors Affecting Durability of Glass-Reinforced Polyester Resins" by A. C. Smith and J. R. Lowry, Plastics Technology, June 1959, pages 42-56. This lack of strength and related properties due to poor copolymerization or curing has been a principal reason why methyl methacrylate has not been more widely used for the curing of unsaturated polyesters.